Abstract

Prebatholithic rocks of the southwest Sierra Nevada foothills contain a 125-km-long northwest-trending disrupted and metamorphosed ophiolite belt. Much of the belt consists of a serpentinite-matrix mélange in which the ophiolitic material is dispersed. The mélange matrix consists of schistose serpentinite derived from tectonitic peridotite and sedimentary serpentinite. The tectonic blocks consist mainly of peridotite, gabbro, basalt, chert, ophicalcite, and silica-carbonate rock and are as long as several kilometres. Within the tectonic blocks relict primary features such as bedding, pillows, dikes, and cumulate layering remain. The mélange represents disrupted and internally mixed oceanic lithosphere of latest Paleozoic to possibly earliest Mesozoic age.

Outcrop mapping of the mélange reveals clustering of blocks into several lithologic associations. The associations are defined as mélange units, which appear to represent the vestiges of once-intact ocean-floor sections. Three types of mélange units have been recognized: (1) peridotite-gabbro units; (2) gabbro-basalt units; and (3) peridotite-chert-basalt units. The first two units represent crust and upper mantle whose deformation and metamorphic history began at the site of ocean-floor genesis. The third unit represents abnormal crust composed of ultramafic protrusions and interbedded sedimentary serpentinite, ophicalcite, chert, and pillow lava. Protrusive activity also started at the site of ocean-floor genesis and continued for an extended time after sea-floor–spreading transport of the ophiolite belt away from the genesis site. Genesis was at an oceanic spreading center that was cut by a transverse fracture zone. Ocean-floor mélange developed along the fracture zone by the combined effect of protrusive acitivity and wrench faulting. Emplacement of the fracture-zone complex (ophiolite belt) resulted from large-scale wrench faulting that truncated the ancient continental margin and juxtaposed the complex against the modified margin.

During transport to the continental margin, a chert-argillite oliostostrome complex was shed across the ophiolite belt. The olistostromes carried limestone blocks with fauna exotic to North America. Once in the vicinity of the continental margin, the ophiolite belt served as basement for continent-derived submarine-fan deposits and island-arc volcanic rocks, both of Late Triassic to Middle Jurassic age. Deformation of these strata along with their ophiolitic basement continued along the older fracture zone trends. The strata now exist as highly deformed depositional remnants above serpentinite mélange.

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